Method of operating and cleaning an apparatus for heat treating a liquid product

ABSTRACT

Apparatus for heating-treating a liquid product comprising series-connected multi-tube heat-exchangers such as heating-up heat-exchangers 6, 12, and high temperature heat-exchanger 14 and cooling-down heat-exchangers 17, 18, 19, the heating-up heat-exchangers and cooling-down heat-exchangers being interconnected by means of a pipe system 25, 26; 34 for regenerative heat-exchange. The heat-exchangers being connected by means of connection members 13 having separate passages which connect the tubes of one heat-exchanger to those of the next heat-exchanger. The pipe system for regenerative heat-exchange being designed so that the quantity of heat to be transferred can be adjusted in order to adapt said apparatus to varying capacities.

This application is a division of application Ser. No. 567,871 filedJan. 3, 1984, now U.S. Pat. No. 4,584,932.

BACKGROUND OF THE INVENTION

This invention relates to apparatus for heat-treating a liquid product,which product is temporarily subjected to a high temperature, comprisingseries-connected multi-tube heat exchangers, a heating-up heatexchanger, a high-temperature heat-exchanger and a cooling-downheat-exchanger, the heating-up and cooling-down heat-exchangers beinginterconnected by means of a pipe system for regenerative heat-exchange,the apparatus being connected to conduits for supplying and/ordischarging heat and being provided with measuring, monitoring andcontrol equipment.

Apparatus of this kind is frequently used for sterilizing fruit juices,milk products and milk. On the one hand, the product to be treated hasto be kept at a high temperature for a certain time during thetreatment, in order to ensure that contaminating organisms are renderedsufficiently inactive, while on the other hand the time during which theproduct is kept at high temperature must not be too long since otherwiseundesirable changes in the quality and the taste of the product arecaused. This applies particularly to milk and milk products. Goodresults are obtained by heating the product for a short time to therequired high heating temperature and keeping it at that temperature foran accurately maintained short period, and then cooling it rapidly.

EP - A - No. 0 036 124 discloses an apparatus of this kind. Thisapparatus is constructed from a large number of identical tubeheat-exchanger sections which are assembled to form heat-exchangers inwhich the product is respectively preheated, heated and cooled. Eachtube heat-exchanger section consists of a tubular housing accommodatinga number of parallel tubes mounted in the housing by means of two endplates. The product to be treated flows from the common space in frontof one end plate to the common space behind the other end plate via thetubes. A heating and cooling fluid respectively flows in the housingbetween the two end plates. This means that the product to be treated isdivided into separate flows in each heat-exchanger section for takingupand yielding heat respectively and that these flows are combined againat the end of each heat-exchanger section, whereupon the product is fedvia a connecting pipe to the next heat-exchanger section, where it isagain divided and so on.

Consequently, the connecting pipes are always at the average temperatureand it is impossible to check whether one or more tubes of a certainheat exchanger are fouled. When the apparatus is washed out with acleaning agent it will also be impossible to tell whether the tubes withthe most fouling have been fully cleaned. One of the results of this maybe that blockages still present locally will prevent some of thecleaning liquid from being removed, so that this liquid may mix with theproduct during the subsequent heat-treatment. The remaining fouling alsocauses a reduction in the sterilizing efficiency of the apparatus andaccelerated new fouling, so that the cleaning frequency has to beincreased at the expense of heat-treatment operating time. There is alsothe risk that the product will be inadequately heated in some of thetubes so that not all the bacteria are destroyed, and this inadequatelytreated product may again be mixed with product which has been subjectedto the correct treatment.

SUMMARY OF THE INVENTION

It is a main object of this invention to provide an apparatus in whichthese drawbacks are obviated. To this end, according to the invention,the tubes in each multi-tube heat-exchanger are constucted to extendcontinuously without interruption, connecting members being providedbetween the heat-exchangers at least in that part of the apparatus wherethere is a risk of fouling, said connecting members having a number ofseparate passages which connect the tubes of one heat-exchanger to thoseof the next heat-exchanger.

In this way, the product to be treated flows through the apparatus via anumber of separate channels and there is no mixing of the said productflows, at least in the area where the greatest fouling occurs.Preferably, each of the passages of at least the connecting membersituated at the outlet of the heat-exchanger where the maximumtemperature sensor connected to the monitoring and control equipment.

Fouling due to the caking of deposits or the like will occur primarilyin the high-temperature heat-exchanger. By measuring the temperature ofthe product to be treated in each channel, it is possible to checkaccurately whether the product has experienced the correct heattreatment everywhere. It is also possible to tell from the temperaturemeasurement which channel is intensely fouled and when the apparatus hasto be shut down for cleaning.

In order to prevent that in case the apparatus is not working at is fullcapacity the product to be treated is subjected to the high temperaturefor a too long period, according to the invention the pipe system forregenerative heat-exchange is designed so that the heat to betransmitted can be adjusted as desired.

According to a preferred embodiment of the present invention the pipesystem for regenerative heat-exchange consists of a parallel pipecircuit disposed outside both heat exchangers, each section of the heatexchangers being connected to the parallel pipe circuit by means of atleast two conduits, via three-way valves.

By adjusting the three-way valves the intermediate fluid can be ledthrough each section or passed outside each section. The number ofsections which will take part in the regenerative heat-exchange processcan be chosen as desired.

Each heat-exchange section can be connected to the parallel circuit bymeans of a plurality of spaced conduits. In this way also only a portionof said section can be used for regenerative heat-exchange.

The present invention also relates to a method of operating theapparatus according to the invention, due to which method thetemperature, (T_(n)) of the product to be treated is measured in eachchannel near the transition from the high-temperature heat-exchanger tothe coolingdown heat-exchanger the average temperature (T_(g)) of thesetemperatures being determined, while the deviation of this averagetemperature from a required or set temperature (T_(i)) is a measurementfor the supply of heat-carrying fluid to the high-temperatureheat-exchanger, and the deviation of each separately measuredtemperature (T_(n)) from the set temperature (T_(i)) is determined, theapparatus being switched over to cleaning operation if this deviationexceeds certain tolerance limits.

The present invention also relates to a method of cleaning the apparatusaccording to the invention, in which cleaning agent is fed through thevarious channels, heat being supplied in the high-temperatureheat-exchanger and the temperature (T'_(n)) in each channel beingmeasured near the transition from the high-temperature heat-exchanger tothe cooling heat-exchanger, while an average temperature (T'_(g)) iscalculated therefrom, the supply of heat-carrying fluid to thehigh-temperature heat-exchanger exchanger being controlled as a functionof the deviation of the average temperature (T'_(g)) from a required settemperature (T'_(i)) cleaning being continued until the deviation ofeach separately measured temperature (T'_(n)) from the set temperature(T'_(i)) is within certain tolerance limits.

SURVEY OF THE DRAWINGS

FIG. 1 is a schematic plan view of the apparatus according to theinvention;

FIG. 2 is an enlarged-scale detail of the connection between twoelements of the apparatus of FIG. 1;

FIG. 3 is a temperature-time graph of the product to be treated in theapparatus of FIG. 1;

FIG. 4 is a detail of the apparatus of FIG. 1 with a modified pipesystem for regenerative heat-exchange;

FIG. 5 is a temperature-time graph of the product to be treated at alower capacity of the apparatus; and

FIG. 6 is a detail of the apparatus, showing a heat-exchange sectionwith connections to the parallel circuit for regenerative heat-exchange.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown schematically in FIG. 1, the product to be treated, e.g. milk,is fed from a reservoir 1 via a feed pipe 2 containing a three-way valve3 connected to a water supply to enable water to be fed to the apparatusto prevent it from boiling dry in the event of any disturbance in thesupply of product to be treated. The milk is fed by a pump 4 in pipe 2to a pre-heating tube heat-exchanger 6 via an inlet connection 5. Fromthe heat-exchanger 6 the milk is fed via a connection 7 and a pipe 8, toa homogenizer 9 and then under elevated pressure via a pipe 10 and aconnector 11 to a number of interconnected heat-exchanger sections 12'and 12", which together form a heating-up heat exchanger 12 of theapparatus. A connecting member 13 comprising a number of parallelpassages is provided between the heat-exchanger sections 12', 12" andconnects the tubes of the heat-exchanger section 12' to those of thenext heat-exchanger section 12" so as to form a continuous channel forthe product to be treated. The milk coming from the heating-upheat-exchanger 12 is then fed via a connecting member 13 to a hightemperature heat-exchanger 14 also provided with a number of paralleltubes, steam being supplied at the connection 15 and condensatedischarged at the connection 16. From the high-temperatureheat-exchanger 14 the milk is fed via a connecting member 13 to a tubeheat-exchanger section 17' of a cooling-down heat-exchanger 17 of theapparatus, which in this case consists of two heat-exchanger sections17' and 17". From this cooling-down heat-exchanger the milk flows via aconnecting member 13 to the heat-exchanger 18 and then via a connectingmember 13' to a final cooler 19. From cooler 19 the milk is fed via anoutlet connection 20 to the pipe 21 containing a throttle valve 22,whereupon the milk is discharged via three-way valves 23 to one or morefilling devices 24.

In the embodiment of the apparatus shown in FIG. 1, an intermediatefluid is always used for the heat transfer. For example, heat from thecooling-down heat-exchanger is transferred to the heating-up heatexchanger by means of an intermediate fluid flowing in counter-currentthrough a closed circuit shown by broken line 25. A circuit 26 for anintermediate fluid is also shown between the heat-exchanger 18 and thepre-heating heat-exchanger 6. Finally, a cooling fluid 27, such aswater, flows in counter-current through the cooler 19.

A temperature sensor 28 is disposed in each passage of the connectingmember 13 situated between the high-temperature heating heat-exchangerand the cooling heat-exchanger 17, the temperature sensors 28 each beingconnected to a control and monitoring unit 29.

It will be apparent that there is no need for an intermediate fluidbetween the cooling heat-exchanger and the heating-up heat-exchanger andthat the heat can be transferred directly from the treated product tothe product, still requiring treatment. In that case, however, it is notpossible to run the product through the heating-up and cooling-downheat-exchangers in continuous separate channels. This need not of itselfbe a drawback, since the present invention lies primarily in the idea ofpassing the product through separate channels only where the greatestfouling occurs and the greatest fouling will occur in the region of thehigh-temperature heat-exchanger.

In the embodiment illustrated, the product flows through the heating-upheat-exchanger 12, the high-temperature heat-exchanger 14, thecooling-down heat-exchanger 17, the heat-exchanger 18, and the cooler 19in separate channels. To this end, the various heat-exchangers and/orheat-exchanger sections are interconnected by means of a connectingmember 13 (shown on an enlarged scale in detail in FIG. 2).

The left-hand part of said FIG. 2 shows one end of a tube heat-exchangeras used in the apparatus according to the invention. This heat-exchangerconsists of a tubular housing 30 closed at both ends by an end plate 31.The housing contains a number of parallel tubes 32 which lead outthrough the end plates. The tubular housing of the heat-exchanger alsocontains an inlet 33 and an outlet (not shown) for a cooling and heatingfluid respectively. The end plate 31' of connecting member 13 isdisposed directly against the end plate 31 of the heat-exchanger and isprovided with a number of parallel passages which correspond, in respectof positions and dimensions, to those of the tubes of theheat-exchangers for connection thereto. The connecting members may beconstructed so as to correspond completely to the heat-exchangers, i.e.having parallel tubes disposed in a housing and end plates, but withoutthe connections for a cooling and heating fluid respectively. The spacebetween the tubes can in that case be filled with insulating material ifrequired, to form a zone where the product temperature remains constant.

In practice the heat-exchanger sections are spiral in shape and can havea length of appr. 20 to 80 meters.

FIG. 3 shows a diagram of the milk temperature as it passes through thesterilizing apparatus of FIG. 1. The numbers given on this graphcorrespond to the apparatus components (FIG. 1).

As already stated hereinbefore, the tubes will foul during operation andthis fouling will be greatest where the higher temperatures occur. Thisfouling is coupled with a decrease in heat transfer. The temperaturesensor disposed in each separate channel readily indicates when thefouling of one of the tubes becomes unacceptable. If this is the case,the apparatus is switched over to cleaning operation, cleaning agentbeing fed through the separate channels. This cleaning operation canalso be carried out very efficiently by means of the apparatus accordingto the invention, because the temperature in the various channels can bemeasured during cleaning operation as well.

Cleaning is continued until the temperature deviations in the differentchannels are within certain tolerance limits. In this way there isabsolute certainty that each channel has been cleaned completely afterthe cleaning operation. If hard-to-shift contamination occurs in one ormore channels, then the apparatus must of course be opened. Aconsiderable advantage in that case is that the channel or channelsrequiring further treatment is/are accurately known.

FIG. 4 shows a detail of a modified embodiment of the apparatusaccording to the invention in which the pipe system for regenerativeheat-exchange has been designed so as to adjust the heat to betransmitted. If, for example the apparatus does not work at its fullcapacity, there is less product to be treated flowing through the tubesof the heat-exchangers or heat-exchanger sections. In this case theproduct will be subjected to the high temperature for a too long periodwhich affects the quality and the good taste of said product. At a lowercapacity the velocity of the product in the tubes is lower andconsequently the transmitted heat per time unit is higher.

In FIG. 5 the solid line "A" shows a temperature-time graph of theproduct when the apparatus is working on its full capacity. The line "B"shows the temperature-time graph of the product in case the apparatus isworking at half its capacity. It will be apparent that in this case theproduct will have a temperature over 100° C. for a much longer periodthan in the first case.

In this connection it is to be noticed that the highest temperature,which will be reached in the high-temperature heat-exchanger 14 is thesame in both cases. The high-temperature heat-exchanger 14 is of thetype disclosed in EP - A - No. 0 081 256 of Applicant and willautomatically adapt to varying circumstances.

The line "C" in FIG. 5 shows a temperature-time graph of the productwhen the apparatus is working at half its capacity and a portion of theheat-exchangers 6, 12, 17 and 18 has been rendered inoperative. In thiscase the period during which the product temperature exceeds 100° C. issubstantially equal as in case of line "A".

The embodiment of FIG. 4 offers the possibility to adjust theregenerative heat transfer by rendering some portions or sections of theheat-exchangers 6, 12, 17 and 18 inoperative. In stead of the circuits25 and 26 the apparatus of FIG. 4 has a closed parallel pipe circuit 34in which circuit a pump 35 and a pressure accumulator 36 of a known typehas been arranged. Each heat-exchanger or heat-exchanger section isconnected to the parallel circuit 34 by means of at least two conduitseach conduit being connected via a three-way valve. So heat-exchanger 6is connected to the parallel circuit 34 by means of conduits 37, 38 andthree-way valves 39, 40; heat-exchanger section 12' by means of conduits41, 42 and three-way valves 43, 44; heat-exchanger section 12" by meansof conduits 45, 46 and three-way valves 47, 48; heat-exchanger section17' by means of conduits 49, 50 and three-way valves 51, 52;heat-exchanger section 17" by means of conduits 53, 54 and three-wayvalves 55, 56; and heat-exchanger 18 by means of conduits 57, 58 andthree-way valves 59, 60.

By operating the respective three-way valves the intermediate fluid canbe led through a heat-exchanger or heat-exchanger section or in caseless heat has to be transmitted, can be led outside the heat-exchangerthrough the parallel circuit 34. In this way it is possible to shut offeach heat-exchanger or heat-exchanger section for regenerative heattransfer and the quantity of heat to be transmitted for regenerativeheat-exchange can be adjusted as desired.

FIG. 6 shows a detail of a modified embodiment of the apparatusaccording to FIG. 4. In this embodiment each heat-exchanger orheat-exchanger section (for example heat-exchanger 6) is connected tothe parallel circuit 34 by means of four conduits 61, 62, 63 and 64 viathree-way valves 65,66,67,68. This embodiment is particularly suitablein case only a portion of a heat-exchanger section has to be renderedinoperative. In the example shown in FIG. 6 it is possible to render 1/3or 2/3 of the length of the section inoperative. The line "C" in FIG. 5illustrates the temperature-time graph of the product treated in anapparatus which has the possibility as shown in FIG. 6. In case of line"C only the first 1/3 portion of heat-exchanger 6 is operative and thelast 2/3 portion is inoperative. In this inoperative portion there is noheat transfer and this part of line "C" extends horizontally. Also thefirst 2/3 portion of heat-exchanger 12 is inoperative and only the last1/3 portion of heat-exchanger 12 is operative by taking up heat from thefirst 1/3 portion of heat-exchanger 17, whereas the last 2/3 portion ofheat-exchanger 17 is also inoperative, as well as the first 2/3 portionof heat-exchanger 18. Finally the last 1/3 portion of heat-exchanger 18transfers its heat to the first portion of heat-exchanger 6.

It will be apparent that within the scope of the invention there aremany possibilities for adjusting the regenerative heat transfer from thecooling-down part to the heating-up part of the apparatus.

The construction of the pipe system for regenerative heat-exchangeaccording to FIG. 4 and 6 offers the possibility to adapt the apparatusfor changing capacities and making the apparatus usable under allcircumstances, for instance when one of the filling devices 24 isfalling out and the apparatus has to work on half its capacity.

What is claimed is:
 1. A method for heat-treating a liquid product inwhich the product is temporarily subjected to a high temperature bypumping of the product through separate flow, product dividing, pluralmulti-tube heat exchangers each of which is connected to the other inseries, comprising the steps of:(A) initially passing the liquid productthrough a first heating-up heat exchanger (B) thereafter exposing theheated-up product of phase (A) to high temperature heat-treating bymeans of a heat-exchanger having external heat supply (C) finallyexposing the highly heated product of phase (B) to a cooling downheat-exchanger; wherein the source of heat for the first heating-upphase (A) being regenerative heat, derived from heat which has beenremoved from the product in the cooling-down phase (C), and furtherwherein the product being passing through series-connected multi-tubetube-insulating connectors, disposed at least between respective heatexchangers of phases (B) and (C), is maintained substantiallycontinuously in separate flows and thus divided by individual tubesthroughout the liquid product treatment (D) measuring the temperature(T_(n)) in each of the respective passages of the connector at thetransition from the high-temperature heat-exchanger to the cooling-downheat-exchanger, and determining thereby the average temperature (T_(g))of these temperatures wherein the deviation of this average temperaturefrom a required or set temperature (T_(i)) is a measurement of thesupply of heat-carrying fluid to the high-temperature heatingheat-exchanger, and the deviation of each separately measured producttemperature (T_(n)) from the set temperature (T_(i)) is determined; (E)switching over to the cleaning operation if this last named deviationshall exceed certain tolerance limits.